I was arguing with one of my photographer friends earlier. He said he didn't like it when the cosplayer has no poses in mind, and that got me thinking. How important is it for a photographer to know how to pose people?

I think a photographer should be able to pose the model, especially if they ask for a shoot. Even if the cosplayer asks for a shoot, I feel that the photographer should still have some general pose ideas.

I personally like it when the photographer has posing suggestions. I also like it when they adjust my current pose slightly (lift your head more, turn your shoulders this way...)

My favorite photographers that i've worked with can pose me, which makes the shoot flow easier. I usually have pose ideas of my own too

I think it's a meet-halfway kind of thing. If you're wearing the costume, you should know the character well enough to know how to pose it.
At the same time, if a photographer is hired to do a photoshoot, I think it's beneficial if they do some research before the shoot so they have an idea of key poses for that character. Part of their job, after all, isn't just pointing and clicking, but also capturing the character.

If the cosplayer brings a few reference images with them to the photoshoot, I think it's an all 'round win.